Preparation process of weather-resistant reinforced carton paperboard based on multi-layer high polymer composite modification

Abstract

The application discloses a paper box paperboard weather resistance reinforcing preparation process based on multilayer high polymer composite modification, relates to the technical field of multilayer composite materials, and comprises the following steps: preparing a low eutectic solvent by mixing choline chloride and oxalic acid, and adding biomass to obtain mixed slurry; taking kraft paper as a core layer, coating an intermediate layer and a surface layer precursor on both sides, and stacking to obtain a blank; performing segmented hot pressing, forming a thickness temperature gradient by using a temperature difference of a pressing plate, and in-situ generating a functional structure of each layer; performing pressure maintaining cooling and cutting and trimming to obtain a weather resistance gradient composite paperboard; the biomass is dissociated by the low eutectic solvent, the fiber skeleton is reserved, and the utilization rate of waste is improved; the gradient structure of a surface layer hydrophobic layer, an intermediate layer bonding layer and a core layer rigid layer is formed by gradient hot pressing, the layers are combined in-situ without glue; the hot pressing and cooling promote the generation of the functional layer, enhance the water resistance, moisture resistance and strength, eliminate internal stress, prevent cracking, and guarantee the use stability in complex environments.

Description

Technical Field

[0001] This invention relates to the field of multilayer composite material technology, specifically to a process for preparing weather-resistant reinforced cardboard boxes based on multilayer polymer composite modification. Background Technology

[0002] With the continuous development of the logistics and packaging industry, cardboard, as the most widely used packaging substrate, is facing increasingly stringent performance requirements, demanding higher weather resistance, higher strength, and lower carbon footprint. Traditional cardboard primarily uses pure wood pulp fiber as its raw material, relying heavily on forest resources. This has led to rising raw material costs and resource consumption pressures. Furthermore, ordinary cardboard is prone to strength degradation and deformation under humid conditions and long-term storage, making it difficult to meet the demands of complex distribution environments. Against this backdrop, utilizing agricultural waste and other biomass resources to replace some wood pulp raw materials, and improving the overall performance of cardboard through structural design and component modification, has become an important research direction in the packaging materials field. Simultaneously, the exploration of green processing technologies is ongoing. Novel treatment systems, such as low-eutectic solvents, with their mild reaction conditions and good component selectivity, are gradually being applied to the pretreatment and modification of biomass fibers, providing new technological pathways for the preparation of biomass-based packaging materials.

[0003] Existing technologies for weather-resistant strengthening and biomass modification of paperboard still face several technical bottlenecks. Conventional paperboard waterproofing modification often relies on surface coating with chemical waterproofing agents or lamination. These methods depend on externally added chemical additives, increasing environmental treatment costs in the production process. Furthermore, the coating and paper base are only physically bonded, making them prone to peeling and waterproofing failure under repeated bending and fluctuating temperature and humidity conditions, failing to achieve long-term weather resistance. Traditional biomass fibers used in paperboard production often employ high-temperature, strong-alkali cooking processes to purify cellulose. This process is energy-intensive, generates large amounts of wastewater, and severely damages the fiber's natural skeletal structure, leading to a decrease in the mechanical stiffness of the formed material, making it unsuitable for direct use as a core layer support material. Multilayer composite paperboard is generally prepared using a layered adhesive bonding process, with layers bonded by external adhesives. This results in limited interfacial bonding strength, making interlayer delamination prone to occur under moisture and stress. Additionally, the lack of gradient matching in the functional design of each layer makes it difficult to simultaneously meet the multiple requirements of surface hydrophobicity, intermediate layer adhesion, and core layer rigidity, thus hindering the overall performance improvement of composite paperboard. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a weather-resistant strengthening process for cardboard based on multi-layer polymer composite modification. This process selectively dissociates hemicellulose and lignin in agricultural waste biomass using a choline chloride-oxalic acid eutectic solvent, while retaining the cellulose fiber skeleton. Using sulfate kraft paper as the core layer, a middle layer precursor containing polylactic acid and a concentrated surface layer precursor are sequentially coated. Through segmented gradient hot pressing conversion, the temperature difference of the pressing plate is used to form a temperature gradient in the thickness direction, which allows the surface lignin to polymerize in situ to form a hydrophobic film, the middle layer polylactic acid and hemicellulose to form an interpenetrating network, and the core layer to maintain a highly crystalline fiber skeleton. Finally, cooling and shaping yields a composite cardboard with no added adhesives and a three-layer functional gradient.

[0005] To solve the above-mentioned technical problems, this invention provides the following technical solution: a weather-resistant strengthening process for cardboard boxes based on multi-layer polymer composite modification, the specific steps of which are as follows: S100, Preparation of eutectic solvent: Weigh choline chloride and oxalic acid and put them into a closed reaction vessel and heat and stir until a uniform and transparent homogeneous liquid phase is formed to obtain choline chloride-oxalic acid eutectic solvent. S200, Preparation of mixed slurry: Take the crushed and sieved agricultural waste biomass, add it to the choline chloride-oxalic acid eutectic solvent, stir at a constant temperature until the biomass components are fully swollen and dispersed, and the hemicellulose and lignin are initially dissociated to obtain a uniform and stable biomass mixed slurry; S300, Layered Structure Assembly: Using sulfate kraft paper as the core substrate, intermediate layer precursor and surface layer precursor are coated sequentially on both sides of the core substrate. Both types of precursors are prepared based on the aforementioned biomass mixed pulp. After being stacked and arranged, a composite preform is obtained. S400 Gradient Hot Pressing Conversion: The composite blank is placed in a flat hot press and subjected to segmented heating and pressing. A temperature gradient in the thickness direction is formed by controlling the temperature difference of the pressing plate, and the dissolution of biomass components and the in-situ generation of functional structures of each layer are completed simultaneously to obtain hot-pressed composite board. S500 Cooling, Shaping and Finishing: The hot-pressed composite board is cooled to room temperature at a constant speed under constant pressure. After the pressure is released, the edges are trimmed to the standard size to obtain the weather-resistant gradient composite paperboard product.

[0006] Furthermore, in the preparation of S100 and the eutectic solvent, the ratio of choline chloride to oxalic acid is in the range of 1:0.8 to 1:1.2, the container heating temperature is in the range of 70℃ to 90℃, the stirring speed is in the range of 200r / min to 300r / min, and the stirring time is in the range of 20min to 40min. Choline chloride acts as a hydrogen bond acceptor and oxalic acid acts as a hydrogen bond donor, forming a stable eutectic system through hydrogen bonding. This liquid phase can directionally break the hydrogen bonds inside the biomass fiber, and has the ability to selectively dissolve hemicellulose and lignin. The dissolution intensity increases synchronously with the system temperature.

[0007] Furthermore, in the S200 process of preparing the mixed slurry, the agricultural waste biomass is one or more combinations of corn stalks, bamboo shavings, and wheat straw. After being crushed, the biomass is sieved to a particle size of 40-60 mesh. The solid-liquid ratio of biomass to the eutectic solvent is 1:7 to 1:9. The constant temperature stirring temperature is 75℃ to 85℃, and the stirring time is 25 min to 35 min. Under these conditions, only hemicellulose and lignin are dissociated, while the long cellulose fiber skeleton is completely preserved. The dissociated components are uniformly dispersed in the solvent system, ensuring uniform material distribution in the subsequent coating and molding stage.

[0008] Furthermore, in the S300 layered structure assembly, the core layer substrate uses a quantitative amount of 200g / m³. 2 ~250g / m 2 The sulfate kraft base paper has a cellulose content of not less than 85% in the substrate.

[0009] Furthermore, in the S300 layered structure assembly, the intermediate layer precursor is prepared by compounding polylactic acid micropowder with biomass mixed slurry, the mass ratio of polylactic acid micropowder to biomass mixed slurry is 1:3 to 1:5, and the single-sided coating amount of the intermediate layer precursor is 80 g / m². 2 ~120g / m 2 Polylactic acid micropowder is uniformly dispersed in biomass mixed slurry. After coating, it fills the space between the core substrate and the surface precursor. During hot pressing, it can gradually melt as the temperature rises and fully contact and entangle with the hemicellulose component in the slurry.

[0010] Furthermore, in the S300 layered structure assembly, the surface precursor is a biomass mixed slurry that has been concentrated under reduced pressure. The solid content of the concentrated slurry is 30%–35%, and the single-sided coating amount of the surface precursor is 30 g / m². 2 ~50g / m 2 This ensures sufficient lignin components on the surface, allowing for the formation of a continuous and dense hydrophobic film.

[0011] Furthermore, in the S400 gradient hot pressing conversion, the segmented heating and pressurization process is divided into two processing stages. The temperature range of the first processing stage is 95℃~105℃, the pressure range is 1.5MPa~2.5MPa, and the holding time range is 4min~6min. In the first processing stage, the low temperature and low pressure environment drives the eutectic solvent to penetrate and diffuse along the thickness direction of the blank, selectively dissolving hemicellulose and lignin inside the fiber. The dissolved components migrate and accumulate in a directional manner to the surface and intermediate layers, and the compaction eliminates the interlayer voids and strengthens the interlayer interface adhesion.

[0012] Furthermore, in the S400 gradient hot-pressing conversion, the processing temperature of the second processing stage of segmented heating and pressurization is 135℃~145℃, the processing pressure is 4MPa~6MPa, and the holding time is 8min~12min; during the hot pressing process, the temperature gradient in the thickness direction is achieved by controlling the temperature difference of the press plate, and the surface temperature of the composite blank is 10℃~15℃ higher than the core temperature; the temperature gradient causes the surface layer to reach a higher reaction temperature, promoting the in-situ polymerization of lignin in the surface layer to form a dense hydrophobic film; the temperature of the intermediate layer promotes the full melting of polylactic acid, which intertwines with the hemicellulose component to form an interpenetrating network; the core layer temperature is lower, and the cellulose retains a highly crystalline fiber skeleton structure.

[0013] Furthermore, in the S500 cooling and shaping process, the cooling pressure is maintained at 1.5MPa to 2.5MPa, and uniform cooling is achieved using circulating water cooling. After cooling to below 25°C, the pressure is released and the board is removed, and the edges are trimmed to the standard size. The pressure-holding cooling process allows the functional structure of each layer to be solidified and shaped under stable pressure, avoiding the internal stress caused by a sudden drop in temperature that could lead to cracking at the interlayer interface. At the same time, it maintains the crystalline structure and overall dimensional stability of the cellulose skeleton, ensuring the mechanical properties and appearance flatness of the finished paperboard.

[0014] Furthermore, in the S500 cooling and shaping process, the weather-resistant gradient composite paperboard product forms a three-layer gradient structure from the outside to the inside: a hydrophobic surface layer, an adhesive intermediate layer, and a rigid core layer. The hydrophobic surface layer is a lignin-based polymer film generated by in-situ hot pressing polymerization. The adhesive intermediate layer is an interpenetrating network structure formed by hemicellulose and polylactic acid. The rigid core layer is a cellulose fiber skeleton with high crystallinity. The three-layer structure forms a chemical bonding interface through in-situ transformation without the addition of external adhesives. The overall structure is continuous and the functions are matched layer by layer.

[0015] Compared with existing technologies, this process for preparing weather-resistant cardboard based on multilayer polymer composite modification has the following advantages: I. This invention utilizes a low-eutectic solvent system to gently dissociate agricultural waste biomass. While preserving the long cellulose fiber skeleton, it achieves the directional dissociation and uniform dispersion of hemicellulose and lignin, obtaining a biomass-based slurry directly usable for coating without complex purification processes. This effectively improves the resource utilization efficiency of agricultural waste and reduces energy consumption and pollution emissions in raw material preparation. Simultaneously, relying on layered coating and gradient hot-pressing conversion processes, and with the help of temperature gradient control along the thickness direction, the surface hydrophobic film is generated simultaneously within the same blank, the intermediate interpenetrating network is constructed, and the core fiber skeleton is preserved. This results in a gradient structure with progressively matching functions from the outside to the inside of the paperboard. The interfaces of each layer form a stable bond through in-situ reactions, eliminating the need for additional adhesives and avoiding the environmental hazards and interface peeling risks associated with adhesives. This comprehensively improves the overall structural stability and functional synergy of the paperboard.

[0016] II. This invention employs a processing method combining segmented heating and pressurization gradient hot pressing with pressure holding and cooling for shaping. Utilizing solvent penetration and component migration during the low-temperature, low-pressure stage, interlayer compaction and directional enrichment of functional components are first achieved. Then, the in-situ transformation of the functional structures of each layer is realized through the high-temperature, high-pressure stage. This results in a continuous, dense hydrophobic barrier on the surface layer, a strong and resilient bonding transition structure in the middle layer, and a highly crystalline, rigid support system in the core layer. The synergistic effect of these three elements simultaneously enhances the water resistance and moisture-proof properties of the paperboard, as well as its overall mechanical strength, effectively addressing the shortcomings of traditional paperboard such as decreased strength and insufficient weather resistance when exposed to water. Furthermore, the constant-pressure, uniform-rate cooling shaping method eliminates internal stress generated during processing, preventing interlayer cracking and dimensional deformation, ensuring the smoothness of the finished paperboard's appearance and long-term stability, and adapting to the packaging needs of complex storage and transportation environments.

[0017] Other advantages, objectives and features of the invention will be set forth in part in the description which follows, and in part will be apparent to those skilled in the art from the following examination or study, or may be learned from the practice of the invention. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are merely some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without any creative effort.

[0019] Figure 1 This is a framework diagram of the preparation process for weather-resistant strengthening of cardboard boxes and paperboard based on multilayer polymer composite modification; Figure 2 This is a flowchart of the preparation process for weather-resistant strengthening of cardboard boxes and paperboard based on multilayer polymer composite modification. Detailed Implementation

[0020] To further illustrate the technical means and effects of the present invention in achieving its intended purpose, the following detailed description of the specific implementation methods, structures, features, and effects of the present invention, in conjunction with the accompanying drawings and preferred embodiments, is provided below.

[0021] Example 1: This embodiment provides a weather-resistant strengthening process for cardboard boxes based on multilayer polymer composite modification, such as... Figure 1 As shown, the specific implementation process is as follows: Preparation of the eutectic solvent: Weigh choline chloride and oxalic acid into a sealed reaction vessel at a mass ratio of 1:0.8. Start the heating and stirring apparatus, setting the vessel temperature to 70℃ and the stirring speed to 200 r / min, and continue stirring for 20 min. During stirring, keep the vessel sealed to prevent solvent evaporation and the entry of external impurities until a homogeneous and transparent liquid phase is formed, yielding the choline chloride-oxalic acid eutectic solvent. This eutectic system uses choline chloride as a hydrogen bond acceptor and oxalic acid as a hydrogen bond donor. The two combine through hydrogen bonding to form a stable liquid phase, which can directionally disrupt the internal hydrogen bonds of biomass fibers, selectively dissolving hemicellulose and lignin. The solubility increases synchronously with the system temperature.

[0022] Preparation of the mixed slurry: Corn stalks were crushed and then sieved to obtain biomass powder with a particle size of 40 mesh. The sieved biomass powder was added to the previously prepared choline chloride-oxalic acid eutectic solvent, with a solid-liquid ratio of 1:7. The mixture was placed in a constant-temperature stirring device, and the temperature was controlled at 75°C for 25 minutes. During the constant-temperature stirring process, the biomass components swelled and dispersed fully, hemicellulose and lignin underwent initial dissociation, the long cellulose fiber skeleton was completely preserved, and the dissociated components were uniformly dispersed in the solvent system, finally obtaining a uniform and stable biomass mixed slurry. This step only achieves the initial dissociation of hemicellulose and lignin, without the need for deep purification, which ensures uniform material distribution in the subsequent coating and molding stages.

[0023] Layered structure assembly. Sulfate kraft paper is selected as the core layer substrate, with a basis weight of 200 g / m³. 2 The cellulose content within the substrate is no less than 85%. Intermediate layer precursors and surface layer precursors are prepared separately, both based on the aforementioned biomass mixed slurry. The intermediate layer precursor is prepared by compounding polylactic acid (PLA) micropowder with the biomass mixed slurry at a mass ratio of 1:3, and continuously stirred until the PLA micropowder is uniformly dispersed in the slurry system. The surface layer precursor is obtained by vacuum concentration of the biomass mixed slurry, resulting in a solid content of 30%. The intermediate layer precursor and surface layer precursor are sequentially coated onto both sides of the core substrate using a roller coating method, with a single-sided coating weight of 80 g / m² for the intermediate layer precursor. 2 The surface precursor coating amount is 30g / m². 2 After coating, the composite preform is obtained by arranging the layers in a stack. The intermediate layer precursor is filled between the core layer substrate and the surface layer precursor, providing a transition medium for subsequent interlayer bonding.

[0024] Gradient hot pressing conversion: The composite preform is placed flat between the plates of a flatbed hot press, and a segmented heating and pressing process is adopted. Different temperatures are set on the upper and lower plates to create a temperature gradient in the thickness direction. The first processing stage is set at 95℃ and 1.5MPa, and held for 4 minutes. In this stage, the low temperature and low pressure environment drives the eutectic solvent to penetrate and diffuse along the thickness direction of the preform, selectively dissolving hemicellulose and lignin inside the fibers. The dissolved components migrate and accumulate directionally to the surface and middle layers. At the same time, the pressure of the plates gradually compacts the preform, eliminating interlayer voids and strengthening the interlayer interface adhesion. In the second processing stage, the temperature is increased to 135℃ and the pressure is increased to 4MPa, and held for 8 minutes. During the hot pressing process, there is a set temperature difference between the upper and lower plates. The surface temperature of the composite preform is 10℃ higher than that of the core layer, forming a temperature gradient in the thickness direction. The temperature gradient allows the surface layer to reach a higher reaction temperature, promoting in-situ polymerization of surface lignin and forming a continuous and dense hydrophobic film layer. The intermediate layer temperature promotes the full melting of polylactic acid, allowing it to fully contact and entangle with the hemicellulose components migrating to the intermediate layer, forming an interpenetrating network structure. The core layer temperature is relatively low, allowing the cellulose to retain its highly crystalline fiber skeleton structure, maintaining the core layer's rigid support capacity. After both processing stages are completed, the dissolution of biomass components and the in-situ generation of functional structures in each layer are achieved simultaneously, resulting in a hot-pressed composite board.

[0025] Cooling, Shaping, and Trimming: The hot-pressed composite board is kept under constant pressure (1.5 MPa) and cooled uniformly using circulating water cooling. After cooling to below 25°C, the pressure is released and the board is removed. The removed board is then trimmed to standard dimensions to obtain the weather-resistant gradient composite paperboard. The pressure-holding and cooling process allows each layer's functional structure to solidify and shape under stable pressure, preventing interlayer interface cracking caused by internal stress from sudden temperature drops. It also maintains the crystalline structure and overall dimensional stability of the cellulose skeleton, ensuring the mechanical properties and smoothness of the finished paperboard. The final paperboard forms a three-layer gradient structure from the outside in: a hydrophobic surface layer, an adhesive intermediate layer, and a rigid core layer. These three layers form chemically bonded interfaces through in-situ transformation, eliminating the need for external adhesives. The overall structure is continuous, and the functions are matched layer by layer.

[0026] Example 2: This embodiment provides a weather-resistant strengthening process for cardboard boxes based on multilayer polymer composite modification, such as... Figure 2 As shown, the specific implementation process is as follows: Preparation of the eutectic solvent: Weigh choline chloride and oxalic acid into a sealed reaction vessel at a mass ratio of 1:1. Start the heating and stirring apparatus, setting the vessel heating temperature to 80℃ and the stirring speed to 250 r / min, and continue stirring for 30 min. During stirring, keep the vessel sealed to prevent solvent evaporation and the entry of external impurities until a homogeneous and transparent homogeneous liquid phase is formed, yielding the choline chloride-oxalic acid eutectic solvent. This eutectic system uses choline chloride as a hydrogen bond acceptor and oxalic acid as a hydrogen bond donor. The two combine through hydrogen bonding to form a stable liquid phase, which can directionally disrupt the internal hydrogen bonds of biomass fibers, exhibiting the ability to selectively dissolve hemicellulose and lignin. The solubility increases synchronously with the system temperature.

[0027] Preparation of the mixed slurry: Corn stalks and bamboo shavings were mixed at a mass ratio of 1:1 and then pulverized. After pulverization, the mixture was sieved to obtain biomass powder with a particle size of 50 mesh. The sieved biomass powder was added to the previously prepared choline chloride-oxalic acid eutectic solvent, with a solid-liquid ratio of 1:8. The mixture was placed in a constant-temperature stirring device, and the temperature was controlled at 80℃ for 30 minutes. During the constant-temperature stirring process, the biomass components swelled and dispersed fully, hemicellulose and lignin underwent initial dissociation, the long cellulose fiber skeleton was completely preserved, and the dissociated components were uniformly dispersed in the solvent system, finally yielding a uniform and stable biomass mixed slurry. This step only achieves the initial dissociation of hemicellulose and lignin, without the need for deep purification, which ensures uniform material distribution in the subsequent coating and molding stages.

[0028] Layered structure assembly: Sulfate kraft paper is selected as the core layer substrate, with a basis weight of 225 g / m³. 2 The cellulose content within the substrate is no less than 85%. Intermediate layer precursors and surface layer precursors are prepared separately, both based on the aforementioned biomass mixed slurry. The intermediate layer precursor is prepared by compounding polylactic acid (PLA) micropowder with the biomass mixed slurry at a mass ratio of 1:4, and continuously stirred until the PLA micropowder is uniformly dispersed in the slurry system. The surface layer precursor is obtained by vacuum concentration of the biomass mixed slurry, resulting in a solid content of 32%. The intermediate layer precursor and surface layer precursor are sequentially coated on both sides of the core substrate using a roller coating method, with a single-sided coating weight of 100 g / m² for the intermediate layer precursor. 2 The surface precursor coating amount is 40g / m². 2 After coating, the composite preform is obtained by arranging the layers in a stack. The intermediate layer precursor is filled between the core layer substrate and the surface layer precursor, providing a transition medium for subsequent interlayer bonding.

[0029] Gradient hot pressing conversion: The composite preform is placed flat between the plates of a flatbed hot press, and a segmented heating and pressing process is adopted. Different temperatures are set on the upper and lower plates to create a temperature gradient in the thickness direction. The first processing stage is set at 100℃ and 2MPa, and held for 5 minutes. In this stage, the low temperature and low pressure environment drives the eutectic solvent to penetrate and diffuse along the thickness direction of the preform, selectively dissolving hemicellulose and lignin inside the fibers. The dissolved components migrate and accumulate directionally to the surface and middle layers. At the same time, the pressure of the plates gradually compacts the preform, eliminating interlayer voids and strengthening the interlayer interface adhesion. In the second processing stage, the temperature is increased to 140℃ and the pressure is increased to 5MPa, and held for 10 minutes. During the hot pressing process, there is a set temperature difference between the upper and lower plates. The surface temperature of the composite preform is 12℃ higher than that of the core layer, forming a temperature gradient in the thickness direction. The temperature gradient allows the surface layer to reach a higher reaction temperature, promoting in-situ polymerization of surface lignin and forming a continuous and dense hydrophobic film layer. The intermediate layer temperature promotes the full melting of polylactic acid, allowing it to fully contact and entangle with the hemicellulose components migrating to the intermediate layer, forming an interpenetrating network structure. The core layer temperature is relatively low, allowing the cellulose to retain its highly crystalline fiber skeleton structure, maintaining the core layer's rigid support capacity. After both processing stages are completed, the dissolution of biomass components and the in-situ generation of functional structures in each layer are achieved simultaneously, resulting in a hot-pressed composite board.

[0030] Cooling, Shaping, and Trimming: The hot-pressed composite board is kept under constant pressure (2 MPa) and cooled uniformly using circulating water cooling. After cooling to below 25°C, the pressure is released and the board is removed. The removed board is then trimmed to standard dimensions to obtain the weather-resistant gradient composite paperboard. The pressure-holding and cooling process allows each layer's functional structure to solidify and shape under stable pressure, preventing interlayer interface cracking caused by internal stress from sudden temperature drops. It also maintains the crystalline structure and overall dimensional stability of the cellulose skeleton, ensuring the mechanical properties and smoothness of the finished paperboard. The final paperboard consists of a three-layer gradient structure from the outside in: a hydrophobic surface layer, an adhesive intermediate layer, and a rigid core layer. These three layers form chemically bonded interfaces through in-situ transformation, eliminating the need for external adhesives. The overall structure is continuous, and the functions are matched layer by layer.

[0031] Example 3: This embodiment provides a weather-resistant strengthening process for cardboard boxes based on multilayer polymer composite modification. The specific implementation process is as follows: Preparation of the eutectic solvent: Weigh choline chloride and oxalic acid into a sealed reaction vessel at a mass ratio of 1:1.2. Start the heating and stirring apparatus, setting the vessel heating temperature to 90℃ and the stirring speed to 300 r / min, and continue stirring for 40 min. During stirring, keep the vessel sealed to prevent solvent evaporation and the entry of external impurities until a homogeneous and transparent homogeneous liquid phase is formed, yielding the choline chloride-oxalic acid eutectic solvent. This eutectic system uses choline chloride as a hydrogen bond acceptor and oxalic acid as a hydrogen bond donor. The two combine through hydrogen bonding to form a stable liquid phase, which can directionally disrupt the internal hydrogen bonds of biomass fibers, selectively dissolving hemicellulose and lignin. The solubility increases synchronously with the system temperature.

[0032] Preparation of the mixed slurry: Wheat straw was pulverized and then sieved to obtain biomass powder with a particle size of 60 mesh. The sieved biomass powder was added to the previously prepared choline chloride-oxalic acid eutectic solvent, with a solid-liquid ratio of 1:9. The mixture was placed in a constant-temperature stirring device, and the temperature was controlled at 85°C for 35 minutes. During the constant-temperature stirring process, the biomass components swelled and dispersed fully, hemicellulose and lignin underwent initial dissociation, the long cellulose fiber skeleton was completely preserved, and the dissociated components were uniformly dispersed in the solvent system, finally obtaining a uniform and stable biomass mixed slurry. This step only achieves the initial dissociation of hemicellulose and lignin, without the need for deep purification, which ensures uniform material distribution in the subsequent coating and molding stages.

[0033] Layered structure assembly: Sulfate kraft paper is selected as the core layer substrate, with a basis weight of 250 g / m³. 2 The cellulose content within the substrate is no less than 85%. Intermediate layer precursors and surface layer precursors are prepared separately, both based on the aforementioned biomass mixed slurry. The intermediate layer precursor is prepared by compounding polylactic acid (PLA) micropowder with the biomass mixed slurry at a mass ratio of 1:5, and continuously stirred until the PLA micropowder is uniformly dispersed in the slurry system. The surface layer precursor is obtained by vacuum concentration of the biomass mixed slurry, resulting in a solid content of 35%. The intermediate layer precursor and surface layer precursor are sequentially coated on both sides of the core substrate using a roller coating method, with a single-sided coating weight of 120 g / m² for the intermediate layer precursor. 2 The surface precursor coating amount is 50g / m². 2 After coating, the composite preform is obtained by arranging the layers in a stack. The intermediate layer precursor is filled between the core layer substrate and the surface layer precursor, providing a transition medium for subsequent interlayer bonding.

[0034] Gradient hot pressing conversion: The composite preform is placed flat between the plates of a flatbed hot press, and a segmented heating and pressing process is adopted. Different temperatures are set on the upper and lower plates to create a temperature gradient in the thickness direction. The first processing stage is set at a temperature of 105℃ and a pressure of 2.5MPa, held for 6 minutes. In this stage, the low temperature and low pressure environment drives the eutectic solvent to penetrate and diffuse along the thickness direction of the preform, selectively dissolving hemicellulose and lignin inside the fibers. The dissolved components migrate and accumulate directionally to the surface and middle layers. At the same time, the pressure of the plates gradually compacts the preform, eliminating interlayer voids and strengthening the interlayer interface adhesion. In the second processing stage, the temperature is increased to 145℃ and the pressure is increased to 6MPa, held for 12 minutes. During the hot pressing process, there is a set temperature difference between the upper and lower plates. The surface temperature of the composite preform is 15℃ higher than that of the core layer, forming a temperature gradient in the thickness direction. The temperature gradient allows the surface layer to reach a higher reaction temperature, promoting in-situ polymerization of surface lignin and forming a continuous and dense hydrophobic film layer. The intermediate layer temperature promotes the full melting of polylactic acid, allowing it to fully contact and entangle with the hemicellulose components migrating to the intermediate layer, forming an interpenetrating network structure. The core layer temperature is relatively low, allowing the cellulose to retain its highly crystalline fiber skeleton structure, maintaining the core layer's rigid support capacity. After both processing stages are completed, the dissolution of biomass components and the in-situ generation of functional structures in each layer are achieved simultaneously, resulting in a hot-pressed composite board.

[0035] Cooling, Shaping, and Trimming: The hot-pressed composite board is kept under constant pressure (2.5 MPa) and cooled uniformly using circulating water cooling. After cooling to below 25°C, the pressure is released and the board is removed. The removed board is then trimmed to standard dimensions to obtain the weather-resistant gradient composite paperboard. The pressure-holding and cooling process allows each layer's functional structure to solidify and shape under stable pressure, preventing interlayer interface cracking caused by internal stress from sudden temperature drops. It also maintains the crystalline structure and overall dimensional stability of the cellulose skeleton, ensuring the mechanical properties and smoothness of the finished paperboard. The final paperboard forms a three-layer gradient structure from the outside in: a hydrophobic surface layer, an adhesive intermediate layer, and a rigid core layer. These three layers form chemically bonded interfaces through in-situ transformation, eliminating the need for external adhesives. The overall structure is continuous, and the functions are matched layer by layer.

[0036] Comparative example: This comparative example uses a conventional surface sizing process to prepare water-resistant paperboard. The specific implementation process is as follows.

[0037] The quantitative value was selected as 225 g / m 2 The substrate is made of sulfate-treated kraft paper with a cellulose content of not less than 85%. A commercially available styrene-acrylic waterproof sizing agent is applied to both sides of the substrate using a roller coating method, with a single-sided coating amount of 40 g / m². 2After coating, the substrate is placed flat in a flatbed hot press. The hot pressing temperature is set to 120°C and the hot pressing pressure is 2MPa. After holding the temperature and pressure for 10 minutes, the substrate is removed and allowed to cool naturally to room temperature. It is then cut and trimmed to the same standard size as in the example to obtain the control paperboard sample.

[0038] This comparative example did not use a eutectic solvent to treat the biomass components, did not construct a three-layer gradient structure, and did not employ a segmented gradient hot-pressing process. It only achieved waterproof modification by coating the surface with a waterproofing agent and was used to compare the performance with the product prepared by the process of this invention.

[0039] To verify the overall performance of the paperboard samples obtained in the above embodiments and comparative examples, performance tests were conducted on each group of samples under uniform conditions. The test items included bursting strength, bursting strength retention rate after 24 hours of immersion in water, interlayer bond strength, surface water contact angle, and longitudinal stiffness. The test results are shown in the table below:

[0040] The test results show that the weather-resistant gradient composite paperboards prepared in the three embodiments outperform the comparative samples in all performance indicators. The most significant improvement was in the retention rate of burst strength after immersion in water, indicating that the gradient hydrophobic structure constructed in this process effectively blocks moisture intrusion, maintains mechanical properties in humid environments, and mitigates the problem of rapid strength decline in ordinary paperboard after moisture absorption. The improved interlayer bond strength verifies the interface bonding method formed by in-situ reaction, which has better bonding stability than traditional coating and bonding, reducing the risk of interlayer delamination. All three embodiments achieved the expected performance improvement, demonstrating that this process is feasible within the defined parameter range and can stably prepare composite paperboards with both mechanical and weather-resistant properties, meeting the packaging requirements of complex transportation and storage environments.

[0041] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A process for preparing weather-resistant reinforced cardboard based on multi-layer polymer composite modification, characterized in that, The specific steps of this process are as follows: S100, Preparation of eutectic solvent: Weigh choline chloride and oxalic acid and put them into a closed reaction vessel and heat and stir until a uniform and transparent homogeneous liquid phase is formed to obtain choline chloride-oxalic acid eutectic solvent. S200, Preparation of mixed slurry: Take the crushed and sieved agricultural waste biomass, add it to the choline chloride-oxalic acid eutectic solvent, stir at a constant temperature until the biomass components are fully swollen and dispersed, and the hemicellulose and lignin are initially dissociated to obtain the biomass mixed slurry; S300, Layered Structure Assembly: Using sulfate kraft paper as the core substrate, intermediate layer precursor and surface layer precursor are coated sequentially on both sides of the core substrate. Both types of precursors are prepared based on the aforementioned biomass mixed pulp. After being stacked and arranged, a composite preform is obtained. S400 Gradient Hot Pressing Conversion: The composite blank is placed in a flat plate hot press and subjected to segmented heating and pressing. A temperature gradient in the thickness direction is formed by controlling the temperature difference of the pressing plate, and the dissolution of biomass components and the in-situ generation of functional structures of each layer are completed simultaneously to obtain hot-pressed composite board. S500 Cooling, Shaping and Finishing: The hot-pressed composite board is cooled to room temperature at a constant speed under constant pressure. After the pressure is released, the edges are trimmed to the standard size to obtain the weather-resistant gradient composite paperboard product.

2. The preparation process for weather-resistant strengthening of cardboard boxes based on multi-layer polymer composite modification according to claim 1, characterized in that, In the preparation of S100 and the eutectic solvent, the ratio of choline chloride to oxalic acid is in the range of 1:0.8 to 1:1.2, the container heating temperature is in the range of 70℃ to 90℃, the stirring speed is in the range of 200r / min to 300r / min, and the stirring time is in the range of 20min to 40min. Choline chloride acts as a hydrogen bond acceptor and oxalic acid acts as a hydrogen bond donor, forming a stable eutectic system through hydrogen bonding.

3. The preparation process for weather-resistant strengthening of cardboard boxes based on multi-layer polymer composite modification according to claim 1, characterized in that, In the preparation of the S200 mixed slurry, the agricultural waste biomass is one of corn stalks, bamboo shavings, and wheat straw. The biomass is crushed and screened to a particle size of 40-60 mesh. The solid-liquid ratio of biomass to eutectic solvent is 1:7-1:

9. The constant temperature stirring temperature is 75℃-85℃, and the stirring time is 25-35 minutes.

4. The process for preparing weather-resistant reinforced cardboard based on multi-layer polymer composite modification according to claim 1, characterized in that, In the S300 layered structure assembly, the core layer substrate uses a quantitative 200g / m³ material. 2 ~250g / m 2 The sulfate kraft base paper has a cellulose content of not less than 85% in the substrate.

5. The process for preparing weather-resistant reinforced cardboard based on multi-layer polymer composite modification according to claim 1, characterized in that, In the S300 layered structure assembly, the intermediate layer precursor is prepared by compounding polylactic acid micro powder and biomass mixed slurry. The mass ratio of polylactic acid micro powder to biomass mixed slurry is 1:3 to 1:5, and the single-sided coating amount of the intermediate layer precursor is 80 g / m². 2 ~120g / m 2 .

6. The process for preparing weather-resistant reinforced cardboard based on multi-layer polymer composite modification according to claim 1, characterized in that, In the S300 layered structure assembly, the surface precursor is a biomass mixed slurry concentrated under reduced pressure, with a solid content of 30%–35% after concentration, and the single-sided coating amount of the surface precursor is 30 g / m². 2 ~50g / m 2 .

7. The process for preparing weather-resistant reinforced cardboard based on multi-layer polymer composite modification according to claim 1, characterized in that, In the S400 gradient hot-pressing conversion, the segmented heating and pressurization process is divided into two processing stages. The temperature range of the first processing stage is 95℃~105℃, the pressure range is 1.5MPa~2.5MPa, and the heat and pressure holding time range is 4min~6min.

8. The preparation process for weather-resistant strengthening of cardboard boxes based on multilayer polymer composite modification according to claim 1 or 7, characterized in that, In the S400 gradient hot pressing conversion, the processing temperature of the second processing stage of segmented heating and pressurization is 135℃~145℃, the processing pressure is 4MPa~6MPa, and the heat and pressure holding time is 8min~12min; during the hot pressing process, the temperature gradient in the thickness direction is achieved by adjusting the temperature difference of the pressure plate, and the surface temperature of the composite blank is 10℃~15℃ higher than the core temperature.

9. The process for preparing weather-resistant reinforced cardboard based on multilayer polymer composite modification according to claim 1, characterized in that, During the S500 cooling and shaping process, the pressure is maintained at 1.5MPa to 2.5MPa, and uniform cooling is achieved using circulating water cooling. After cooling to below 25°C, the pressure is released and the board is removed, and the edges are trimmed to the standard size.

10. The preparation process for weather-resistant strengthening of cardboard boxes based on multilayer polymer composite modification according to claim 1, characterized in that, In the S500 cooling and shaping process, the weather-resistant gradient composite paperboard product forms a three-layer gradient structure from the outside to the inside: a hydrophobic surface layer, an adhesive intermediate layer, and a rigid core layer. The hydrophobic surface layer is a lignin-based polymer film generated by in-situ hot pressing polymerization. The adhesive intermediate layer is an interpenetrating network structure formed by hemicellulose and polylactic acid. The rigid core layer is a cellulose fiber skeleton with high crystallinity. The three-layer structure forms a chemical bonding interface through in-situ transformation without the addition of external adhesives. The overall structure is continuous and the functions are matched layer by layer.

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